1. Field of the Invention
This invention relates to a protective device for a wheel bearing of a motor vehicle, which is equipped with a rotational speed measuring device.
2. Relevant Art
The rotational speed of automobile wheel bearings often needs to be measured, e.g., to control an ABS (anti-lock braking system). Conventionally, rotational speed measuring devices consist of a magnetic elastomer (magnetic encoder), which is embedded or integrated into the seal of the antifriction bearing. In order to be able to tap off the magnetic signal from this encoder, the latter is fitted to the outside of the antifriction bearing and connected to the rotating part of the antifriction bearing. Arranged at a short distance in front of latter is a Hall sensor, which evaluates the magnetic pulses from the rotating encoder and in this way determines the rotational speed signal. Both the magnetized encoder and the Hall sensor operate with magnetic parts. In EP 607719 B1, for example, such seals with external magnetic encoders are shown.
Experience has shown that these magnetic encoders or the associated sensors can be contaminated by ferromagnetic particles. These can be, for example, fine iron filings, which are deposited on the encoder over the course of time, or also ore-bearing soil. If too much ferromagnetic contamination is deposited on the magnetic encoder, the functioning of the encoder can be restricted to a great extent. In the event of severe contamination, this can even lead to the rotational speed signal from the wheel bearing no longer being reliably obtainable.
In JP 56 12 08 19, two permanent magnets are shown, which are intended to build up an opposing magnetic field, in order to protect an antifriction bearing against metallic particles from the electric motor. The problem with this arrangement is that the metallic particles which have become lodged between the magnets can no longer be thrown out and thus, given an excessive quantity of contaminants, get into the antifriction bearing.